1. Field of the Invention
The present invention relates to a method and apparatus for enriching the hexavalent chromium in a chromium plating bath. More particularly, the present invention relates to a method and apparatus for enriching the hexavalent chromium content in an operating chrome plating solution with chromium from a spent chromium plating solution.
2. Description of the Prior Art
Typical chromic acid baths are used to electroplate chrome on various surfaces. The baths contain hexavalent chromium. When chromium metal is deposited from the typical solution of chromium acid (e.g., 2.5M chromic acid and 0.025M sulfuric acid, the reduction reaction at the cathode is less than 25 percent efficient. The principal by-product of the process is hydrogen, but some trivalent chromium and other cations are also generated from the reduction.
Over a period of time during operation of such baths, the buildup of trivalent chromium and certain other cations is deleterious to the chromium plating bath. Among the problems caused by the trivalent chromium and other cations are: reduced throwing power which is defined as the ability to initiate plating over an irregular surface; reduced bath conductivity which increases the voltage required and power costs; and changed plate distribution which means that a complex rack designed to produce a uniformly plated surface with a fresh solution may no longer to do. Other deleterious cations typically encountered are iron, copper and aluminum. The effects of the individual cations are additive and some hard chromium platers will set a limit of 15 grams per liter for the total of all cations.
Soluble anodes commonly used to replenish the metal in other plating baths cannot be used in chromium plating as the chromium dissolves as the trivalent cation. Therefore, lead is used as an insoluble anode and it has been found that not as much trivalent would be produced as might be expected or that some of the trivalent was being oxidized back to the hexavalent cation at the lead anode. Platinum and other insoluble anodes do not produce this effect and a great deal of trivalent chromium is produced. It has been found that for trivalent to be oxidized to hexavalent, the lead must be covered with a film of lead oxides and that the oxides are the actual reducing agent. During electrolysis, the lead oxide is constantly being reformed as soon as it is reduced. When a lead anode is first put in service it must be electrolyzed until a film of lead oxide is formed. (Conveniently, the lead oxide film is an unmistakable chocolate brown to almost black color.)
It is recommended that the ratio of the area of the anode to the area of the work being plated be at least 2:1 to avoid build-up of trivalent chromium in the bath. Unfortunately, particularly in chromium plating applied for engineering design reasons as opposed to decorative plating applied for aesthetic reasons, this is frequently not possible and the amount of trivalent chromium in the tank builds up until the bath becomes unusable.
Historically, one of the methods to solve the problem of excessive trivalent (or cationic) contamination was to decant and discard a portion of the solution which is prohibitively expensive today. An environmentally acceptable traditional method was the practice of "dummying" the tank. A procedure where a small cathode was plated with a very large anode area which oxidized more trivalent at the anode than was formed at the cathode. This will reduce the trivalent level, albeit slowly and with a relatively high power cost.
The use of a porous pot surrounding the cathode was developed prior to World War II. The pot confined the trivalent formed more or less to the volume of solution within the pot and it did not mix with the body of the solution which increased the efficacy of the process in comparison with plain dummying. The use of the porous pot dummying is widespread and is typified by the Peger cell. The ceramic pot used by Peger and others is an unglazed ceramic that will withstand the plating solution. The pore size varies but if filled with water, it will weep slowly and is retentive enough for plating solutions that they can be pumped out without excessive influx or can even be physically emptied. However, the pot is not a semi-permeable membrane and back migration of the ions will occur when power is turned off. It is important to empty the contaminated solution within the pot when turning the power off.
In the Peger pot, the cathode is inside the pot and it is claimed that migration of the trivalent into the pot takes place. However, this may be misleading, for while some migration will take place as cations do move towards the cathode, much more than 95 percent of the trivalent inside the pot is caused by the reduction of the solution in the pot. The green color described in the pot is trivalent chromium. Most of the reduction in trivalent that is obtained with the pot is by oxidation on the surface of the lead anodes outside the pot.
In summary, porous ceramic pots are a reasonably efficient device for reducing the level of trivalent chromium in a plating bath, however, the use of this device does not increase the hexavalent chromium concentration in an operating solution, it merely reduces the level of trivalent chromium.
There are a number of patents discussing electrodialysis using a perfluorosulfonic acid cation exchange membrane separating the anolyte from the catholyte. These are effective in oxidizing trivalent chromium and will remove iron and copper from the chromium plating bath. However, the efficiency is poor.
It is desirable to provide a method and apparatus which enriches the hexavalent chromium concentration in an operating chrome plating solution with chromium from a spent chrome plating solution without decanting and discarding a portion of the operating solution. Such method and apparatus is particularly desirable because it reduces the cost of plating, the cost of disposal and is more environmentally friendly.